1. Field of the Invention
This invention relates generally to bearings and specifically to ball, roller and sleeve bearings having solid lubricants which can operate in adverse environments for long times without necessity of repeated application of lubricating oil or grease.
2. Description of the Prior Art
Ball, roller and sleeve bearings are known and have been used in mechanical devices essentially since the beginning of the industrial revolution. While such bearings are serviceable in wide variety of applications and environments, these bearings have disadvantages. One major disadvantage is the need for lubricating oil or grease to provide adequate lubrication between the moving surfaces, so that friction does not cause the bearings to overheat and self-destruct during operation.
Conventional petroleum-based oil and grease lubricants tend to dry out and/or evaporate over time and generally must be replaced periodically for the bearing to function adequately over its design life. To perform such lubrication, workers require access to the bearings; this means downtime for the associated machinery.
Adequately reliable solid lubricated, high temperature bearing systems do not exist, even though such bearings would reduce maintenance and increase permissible operating temperatures and reliability of the associated machinery.
Use of solid lubricants and dry, wear-resistant materials in low and high temperature bearing applications has been attempted, using molybdenum disulfide, graphite and Teflon. The bearing industry markets several types of dry bearing materials based on such molybdenum disulfide, graphite, Teflon and other plastics to be used as solid lubricants. Woven glass fiber-reinforced Teflon bearings are fabricated by bonding a stiff metal backing to a thin composite layer of soft (but solid) lubricating Teflon, reinforced with a hard glass fabric so that very thin film of Teflon lubricates the glass fibers with a minimum of deflection, plastic flow and wear.
Conventional petroleum-lubricated ball bearing-equipped material handling system trolleys are illustrated in the brochure "Ball Bearing Trolleys" published by FMC Corporation, booklet 710105, available from the FMC Corporation Material Handling Systems Division in Chalfont, Pa.
In some applications, it is desirable to have electrical conductivity between the inner and outer bearing races. Conventional oil or grease lubricated bearings typically are not electrically conductive. Generally the lubricating oil or grease, especially if petroleum-based, is electrically insulative. When a thin film of lubricating oil or grease resides on the bearing surfaces, the thin film electrically insulates the contacting surfaces from one another.
One application in which electrically conductive bearings are necessary is electrostatic spray painting. In many industries parts to be electrostatically spray painted are suspended from trolleys traveling along elevated tracks. A charge is applied to the objects to be painted by applying electrical charge to the elevated track on which the trolleys, carrying the objects, travel. To impart the charge to the objects an electrically conductive path must be provided from the track through the trolley to the object to be painted.
In known electrostatic spray painting systems the limiting factor (in providing an electrically conductive path to the object to be painted) is electrical resistance between the moving surfaces of the bearings, which are part of the electrically conductive path between the trolley wheel and free. This resistance results from the presence of electrically insulative lubricating oil or grease film on bearing surfaces.
Bearings conventionally used in applications requiring electrical conductivity have a major disadvantage: When the electrically insulative lubricating oil or grease film is between the bearing moving surfaces, arcing occurs across the film between the surfaces. This arcing pits the case-hardened metal surfaces, leading to early failure of the metal surfaces and consequent bearing failure.
One approach to providing an electrically conductive bearing has been to equip conventional bearing assemblies with a sliding contact finger riding on (or otherwise structurally connected to) the trolley load-carrying structural member affixed to the inner race of the assembly. The contact finger slides along the track on which the trolley assembly rides, providing an electrically conductive path between the track and the trolley carrying the object to be electrostatically spray painted. This approach has not been successful.
In known solid lubricated bearing applications, sporadic catastrophic bearing failures occur. Reliable, self-contained, solid lubricated bearings are needed to reduce maintenance and to facilitate higher operating temperatures that can greatly improve performance and fuel efficiency of machines.